I've been running 2 Sapphire 6950s in xfire for sometime first with a clean install of the 12a drivers then with the 11.1 beta. I did flash my cards bios with the "Unlocked Sapphire file" before installing the 11.1 drivers.
I have been using TRIXX to overclock to 880/1325 which should be safe.

The other night I tried to go back to the 12a drivers because TRIXX worked better with them than the 11a. I did the driver uninstall booted in safe mode ran Driver Sweeper rebooted and tried to install the 12a drivers. The Drivers would not install correctly. I repeated the uninstall process and in safe mode even uninstalled the video adapter.

I have tried with both cards, a single card and have swapped my 2 cards around and no luck.

I also tried to go back to the 11.1 drivers and have the same issue. The Drivers fail to install.

What mobo are you using? One with an AMD chipset? I had a problem the other week where it wouldn't recognise my GPU (when installing CCC) no matter what I did. Re-installing the mobo's chipset drivers fixed it, strangely enough.

I have an ASUS P6T, win7.
My Sapphire 6950s are flashed with the unlocked shaders 6950 bios. I set the bios switches back to position 2 for the default bios settings and they were very stable before I did the first driver uninstall last Friday.

Did you make a Backup (Image) of your Hard drive?
Did you Create a System Restore Point?
Do you have a original copy of your HD 6950 BIOS ?
Does your MotherBoard provide a backup/Dual protection of CMOS.

Please fill in your system specs here and tick the "Show System Specs" box, this can help everyone to identify the problem.

EDIT: lol just spotted what the problem might be...
Boot normally (not safe mode)
Download CCleaner and install it.
go to Control Panel>Device Manager>Display Adapters and uninstall it**
Without rebooting, run CCleaner (remember to select the "registry" tab in CCleaner to clean registry)
Now try to install the drivers. If it still gives you hassle, reboot into safe mode, then make sure your graphics card is uninstalled in Device Manager, run CCleaner and install drivers again.

**I'm guessing that your graphics card hasn't got a yellow triangle/exclamation mark next to your graphics card in Device Manager. I'm guessing that windows is already using a driver, and that's stopping Catalyst from installing.

If none of the above works, take note of the names of the files you're graphics adapter is using from Device Manager (see attachment) and delete them manually after uninstalling your card. Shouldn't come to this though, CCleaner should do it.

I tried all of the above and managed to load the 12a drivers without errors finally. But I'm only getting a score of 4700 in 3Dmark11 where I used to get scores around 8800 back when i first got the cards.

I did have some problems along the way. After uninstalling the cards in device manager the top had the yellow triangle and an error code - "This device cannot start code 10".

I swapped the cards around reset the motherboard bios and installed the complete 12a driver package and got error - it failed to install the drivers. So, I installed the separate 12a driver file as administrator and it finally loaded successfully. I then setup xfire in CCC.

However something is still not right as my xfire 3Dmark score is equal to a single card.

GPU Z shows both cards reaching 100% load however the secondary card seems to run 100% most of the time.

I tried all of the above and managed to load the 12a drivers without errors finally. But I'm only getting a score of 4700 in 3Dmark11 where I used to get scores around 8800 back when i first got the cards.

I did have some problems along the way. After uninstalling the cards in device manager the top had the yellow triangle and an error code on 10.

I swapped the cards around reset the motherboard bios and installed the complete 12a driver package and got error - it failed to install the drivers. So, I installed the separate 12a driver file as administrator and it finally loaded successfully. I then setup xfire in CCC.

However something is still not right as my xfire 3Dmark score is equal to a single card.

Since the secondary card kept runnng at 100% load I disabled xfire in CCC as soon as I hit apply the screen image went out - just snowy noise. I re-booted and the screen went back to normal resolution etc.. Also CCC was back in xFire and the primary card was handling most of the load now and the secondary card was no longer running at 100%.

I ran 3Dmark11 and had an 8723 score so things seem to be working as they should for now...??..??

But i still don't understand why.....

The only thing different was that I installed TRIXX for overclocking a few days before I installed the 11.1 driver and then things went bad when I tried to go back to the 12a driver.